Xiao Xiayun

Palynological evidence for vegetational and climatic changes from the HQ deep drilling core in Yunnan Province, China

The high-resolution pollen study of a 737.72-m-long lake sediment core in the Heqing Basin of Yunnan Province shows that the vegetation and climate of mountains around the Heqing Basin went through six obvious changes since 2.780 Ma B.P. Namely, Pinus forest occupied most mountains around the studied area and the structure of vertical vegetational belt was simple between 2.780 and 2.729 Ma B.P., reflecting a relatively warm and dry climate. During 2.729–2.608 Ma B.P., the areas of cold-temperate conifer forest (CTCF) and Tsuga forest increased and the structure of vertical vegetational belt became clear. According to percentages of tropical and subtropical elements growing in low-altitude regions rifely increased, we speculate that the increase of CTCF and Tsuga forest areas mainly resulted from strong uplift of mountains which provided upward expanding space and growing condition for these plants. Thus, the climate of the low-altitude regions around the basin was relatively warm and humid. Between 2.608 and 1.553 Ma B.P., Pinus forest occupied most mountains around the studied area and forest line of CTCF rose, which reflects a moderately warm-dry climate on the whole. During 1.553–0.876 Ma B.P., the structure of vertical vegetational belt in mountains around the studied area became complicated and the amplitude of vegetational belts shifting up and down enlarged, which implies that the amplitude of climatic change increased, the climatic associational feature was more complex and the climate was moderately cold at a majority of the stage. During 0.876–0.252 Ma B.P., there were all vertical vegetational belts existing at present in mountains around the studied area. The elements of each belt were more abundant and complex than earlier. At different periods in the stage vertical vegetational belts occurred as expanding or shrinking, and alternated each other. The amplitude of vegetational belts shifting up and down was the maximum in the whole section. This change suggests that the amplitude of climatic change was evidently larger than earlier, but the frequency reduced and the climatic associational feature was more complex. From 0.252 Ma B.P. to the present, the most time was characteristic of the expanding of Pinus forest and semi-humid evergreen broad-leaved forest (SEBF) in mountains around the studied area, while expanding time of other vegetational belts was very short, which reflects a smaller amplitude of cold and warm fluctuation. On the basis of the six obvious cycles of vegetational and climatic changes, there were still many times of secondary vegetational successions and climatic oscillations. Based on the above analysis, the forcing mechanism of vegetational succession and climatic change in the Heqing Basin is further discussed. It is primarily considered that main influential factors were exterior factors such as orbital parameters, etc., but the uplift of the Qinghai-Tibet Plateau played a very important function for environmental changes in the Heqing Basin at two times obvious increase of vertical vegetational belts and three climatic transitions.

Environmental changes during early-middle Holocene from the sediment record of the Chaohu Lake, Anhui Province

A typical lake sediment core is obtained from the Chaohu Lake in the lower reaches of the Yangtze River, Anhui Province, China. The timing scale is constrained by AMS 14C dating method. Climate proxies such as pollen and grain size in the core are analyzed to reconstruct the environment changes at this site approximately between 9870 and 2170 cal. a BP. The results indicate that at the research area, the climate in the early-middle Holocene had evolved through 3 stages. From 9870 to 6040 cal. a BP, proxy records show a warm and dry climate with low water levels after the late-glacial period. During this stage, cool and dry events occurred at about 8910 and 6060–6030 cal. a BP. Then, between 6040 and 4860 cal. a BP, the climate was humid and vegtation was more flourishing in the Chaohu Lake Valley. The Holocene Optimum occurred at 5840–5500 cal. a BP in the Chaohu Lake, showing the best condition of water and heat. Elm Decline occurred at the period of 5380–4930 cal. a BP. Since 4860 cal. a BP, the climate was warm and dry through 2170 cal. a BP as shown in both pollen spectrum and grain-size histories. Two obvious dry events occurred in 3760 and 2170 cal. a BP, respectively. At 2170 cal. a BP, the water level of the Chaohu Lake reached the lowest as the lakebed possibly exposed. Such lake sediment observations are consistent with the historical records in this area.

Paleoenvironmental evolution of Heqing basin in Yunnan Province since 2.78 Ma

Based on multi-proxy investigations such as grain size, content of carbonate and loss on ignition of the deep core of the ancient lake in the Heqing basin, the northwest of Yungui Plateau, it is reconstructed for a 2. 78 Ma paleoenvironmental evolution. The result of magnetic stratum indicates that the Heqing lake basin was formed at about 2.78 Ma. Further, the multi-proxy analysis reveals that there had been three major environmental stages in the Heqing basin since 2. 78 Ma, namely the lake basin accumulated water to be a lake in 2. 65 Ma,and the height difference between the mountains and basin increased twice at about 1. 55 Ma and 0.99 Ma. These stages couples well with Qingzang movement phase B, phase C and Kunlun-Huanghe movement respectively.

Pollen records and vegetation and climate changes in Heqing Basin, Yunnan Province since middle Pleistocene

Vegetational succession and climatic change since middle Pleistocene in Heqing Basin in Yunnan Province are discussed according to the sporopollen analysis for the top 155m length core and about 800 kaBP history from Heqing deep drilling core. This core is the first drilling core of Environmental Drilling Programme in Chinese continent. The results show that vegetation in the surrounding mountains in Heqing cathchment experienced many times changes among coniferous forest dominated by Pinus, montane coniferous and broadleaved mixed forest and cool temperate coniferous forest during the period about 800 kaBP to 6. 98 kaBP. From 6. 98 kaBP to today, vegetation has immensely changed, such as forest degraded quickly and vegetation dominated by herbs, when human activities have begun affecting vegetation. Human activities represent disafforestation and planting crops and so on. Climate corresponding to vegetation evolution may be divided into five stages and contains many times cool and warm, dry and humid gyrations since about 800 kaBP.

IMPACT OF ENVIRONMENTAL EVOLUTION AND COASTLINE CHANGE ON THE NEOLITHIC CULTURES IN THE EASTERN PART OF JIANG-HUAI AREA

More than ten neolithic cultural sites have been discovered in the eastern part of Jiang-Huai area,showing a cultural sequence consisting of the Longqiuzhuan culture(7 000?6 300~5 500 aBP),Dawenkou culture(5 500~4 600 aBP)and Longshan(YueShi)culture(4 300~3 450 aBP).During the period of Holocene optimum,due to the joint action of favorable climate,rapid development of the Yangtze and Huai river delta and sea level changes,land was expanded eastward about 7 000 aBP.Multi-channel sand bars and lagoons were developed.As the result,the lowland of LiXiaHe area was formed.The environment provided favorable conditions for the survival of aquatic animals,large mammals and plants and the settlement of human being.During the YueShi cultural stage(3 450 aBP years later),however,the climate became mild and slightly dry,and the Jianghuai area was dominated by salt marshes under frequent tidal flooding,that forced the ancient residents to move away and caused the break of cultural records.It suggests that palaeoenvironmental changes is the main factor to the rise and decline of prehistoric cultures.

A 0.1 ka-year Record of Environmental Evolution in Hongjiannao Lake, Shaanxi Province

The environment evolution in the past nearly 100 years was revealed by the multi--proxy analysis of lithology, grain size, magnetic susceptibility, TOC and geochemical elements from the Core H in Hongjiannao Lake, Shaanxi Province. The result indicates that Hongjiannao Lake was formed at about 1927 AD, characterised by a relatively higher value of Fe/Mn ratio and a lower level of lake productivity and magnetic susceptibility value. During the span between 1952 and 1960, the lake level rose remarkably, as shown by the sharp decrease of clay fraction and the obvious dilution effect on the geochemical elements concentration in sediment. The lithologic character and sedi- ment proxies reflect that the lake has developed into a stable deep--water condition since 1960. The lake formation and its subsequent rapid expansion in the late 1950s related closely to the regional rainfall increases, and the latter also benefited from extensive reclamation of the lowland around the lake. Human activities have also resulted in the higher lake production and the changing of redox condition on sediment--water interface in the recent 40 years. Moreover, the 3 strong dust storm events happened in the late 1930s and 1940s were well distinguished in terms of the features of grain size frequency distributions in the sediment core.

Climatic changes and human activities revealed by pollen records in Lake Taibai, Hubei Province, the middle and lower reaches of the Yangtze River region over the past 1500 years

Pollen analysis and charcoal indices of a 153-cm-long lake sediment core from Lake Taibai (TN1 core) in Hubei Province disclosed seven stages of the vegetational changes in Lake Taibai drainage area over the past 1500 years. During 520-1310AD, the vegetational changes were mainly controlled by the climatic changes and the influence of human activities on vegetation was relatively weak. Stage of 1310-1710AD, the influencing intensity of human activities on vegetation gradually increased, and the stage was a transitional period of main factors from natural control to human activities drive. Since 1710AD, the vegetational changes were controlled by human activities and the climatic signal from the vegetational changes was relatively weak. According to the pollen assemblage, the climatic changes with three warm periods of 520-720AD, 1050-1310AD and since 1950AD and two cold periods of 720-1050AD and 1310-1710AD could be speculated.

First investigation of desert Lake Hongjiannao, Shannxi Province based on its sediment study

By multi-proxy analysis of grain-size, magnetic susceptibility (MS), Fe/Mn and Rb/Sr ratios, TOC and other chemical elements, environmental changes in the Hongjiannao Lake over the past 80 years were discussed. It is revealed that the lake formed during 1928 – 1952 A. D., characterized by relatively high Fe/Mn ratio and lower Rb/Sr ratio, MS value and lake productivity. During this period sandstorms were prevalent with three extremes occurring in 1936, 1939 and 1941 A. D., respectively. In the short time period of 1952 – 1960 A. D., the water level of the lake rose remarkably, as shown by a sharp increase in coarse slit and MS value, as well as the obvious dilution of geochemical element concentrations in lake sediments. The lake expansion is ascribed to both much more rainfall and human activities, especially in about 1958 A. D., when a great deal of channeled-off water from the around swamps was let to inpour into the lake. The lithological characteristics and sediment proxies reflect that the Hongjiannao Lake has already developed into a stable deep lake since the 1960s, characterized by the higher lake productivity and Rb/Sr ratio and lower Fe/Mn ratio. The frequency and intensity of sandstorms have also reduced greatly since the 1960s due to natural conservation activities. The drop of water level since the middle of the 1990s may be related to a warm and dry climate.

Sporopollen Record and Environmental Evolution Since-100 years in Lake Hongjiannao,Shaanxi Province

Environment evolution is revealed since about 100 years in Lake Hongjiannao, Shaanxi Province according to sporopollen analysis, after comparing with lithological character, historical records and instrumental data. The results show that before 1927 the regional climate was very dry in Lake Hongjiannao with typical hungriness vegetation, and the lake did not come into being. Between 1927 and 1938, the dryness degree of climate gradually eased up, lake basin depression began to collect water and lake formed. The vegetation type turned to be hungriness and brushwood-shrubbery vegetation between 1938 and 1960 as the regional climate relatively got better and local environment began to become wet. Under the relative better climate condition Lake Hongjiannao received more amount of water and became a deeper lake than 1927 - 1938; the vegetation type was grassland and brushwood-shrub vegetation. Around 1960 the local environment had the greatest change, as indicated by sporopollen diagram, which showed the superimposed influence of the human activities and precipitation increase in the region. Between 1960 and 2000, climate evidently improved than that of former several stages. The continuous wet environment led to the steady high-stage lake level after short -time widening in about 1960; the corresponding vegetation was typical, with more flourish grassland and brushwood-brush. In recent years, the lake area began to turn relative dry again. The vegetation type was meadow and grassland vegetation dominated by Chenopodiaceae.

The orbital scale evolution of the paleoproductivity reflected by Heqing Core in Yunnan Province

Based on loss on ignition of different parts of Heqing sediment cores, we studied the characteristics of orbital scale evolution of the lake paleoproductivity by using Spectral analysis and Filter methods. It shows that obliquity cycle is stronger than precession cycle in the 2. 78 Ma, due to different phasing relationship between summer insolation in the northern hemisphere and latent heat export in the southern Hemisphere. 15 ka, 10.7 ka and 10 ka are the relatively stronger cycles from 987 ka to 1552 ka, in which 10 ka and 10. 7 ka are semi-precession cycles related to the strengthened 100 ka cycles. Further, there is Mid-Pleistocene Revolution coincident with climatic changes in the evolution of lake paleoproductivity.